Particularly with regard to power devices capable of switching large currents and/or operating at higher voltages, both high breakdown voltages Ubd and low on-resistance Ron are often desired.
Power semiconductor switches often handle voltages differences of more than several 10 V or even several 100 V within the same semiconductor piece. To avoid breakdown during blocking mode, drift zones are used. The dimension of the drift zones depend on the breakdown voltage Ubd, which has to be blocked, but also on the doping level of the semiconductor material. For lateral devices, the size of the drift zone mainly determines the size of the chip and has, thus, a high influence on costs. To keep wafer cost low, the doping level of the wafer material should be low too. Wafer material doped at a level of about 1015/cm3 enables small drift zone dimensions, while still having enough doping concentration too keep also Ron low enough to enable a decent ON-state of the switch.
Small chip size may also be achieved, if the drift zone is arranged in a vertical structure. However, at the edge of such structures again high voltage differences to neighboring structures may occur. Effective isolation between regions of high voltage differences is often required. Such isolations may be realized with oxide filled trenches, so called Shallow Trench Isolations or Deep Trench Isolations. However, a leakage path may occur at the semiconductor-oxide interface. This may hinder the desired blocking behavior.
For these and other reasons there is a need for the present invention.